The present invention relates to the use of adsorption as a means to generate high purity oxygen from air. More particularly, the present invention relates to the separation of oxygen from mixtures of argon and oxygen by the use of an adsorbent selective for argon over oxygen in a pressure swing adsorption technique.
Generally speaking, air comprises carbon dioxide and water vapor and a mixture of about 79% nitrogen, 20% oxygen and 1% argon. It is often desirable to have a gaseous product of relatively pure oxygen such as can be provided from the removal of other gases from air. Adsorbent process techniques using pressure swing adsorption and zeolite molecular sieves have been employed to remove the carbon dioxide, water vapor and nitrogen from air. While the removal of carbon dioxide, water vapor and nitrogen is straightforward, the removal or separation of argon from air using an adsorbent technique is difficult. Without removal of argon, the oxygen enriched product from air has a maximum purity of about 95%, the balance being argon, since the ratio of oxygen to argon in air is approximately 20 to 1.
Argon is difficult to separate from oxygen because argon has adsorptive and diffusive properties almost identical to oxygen. Hence, the generation of oxygen from air by adsorption techniques has heretofore been limited to the generation of oxygen having a purity of 95% or less and the only feasible method of producing high purity oxygen, i.e., greater than 95% pure oxygen, has been by the cryogenic distillation of air. However, cryogenic distillation of air is practical only when large production quantities of oxygen are required. There is a need for high purity oxygen in moderate or small quantities for example, in ferrous metal cutting and welding and in medical use. Conventional adsorption techniques provide oxygen of only about 90% purity. Thus, there remains a need for a novel adsorption technique that can economically provide oxygen of high purity and at moderate to small quantities.
Although pressure swing adsorption processes for gas separation and zeolite molecular sieves have been known, it has heretofore been believed that such techniques were not practical for the separation of argon from oxygen to provide enriched oxygen because argon and oxygen have almost identical adsorptive and diffusive properties. It has now been discovered, however, that the practical separation of argon from oxygen can be achieved by the use of a bed of silver exchanged zeolite type X. Further understanding of the present invention will be had from the following description taken in conjunction with the accompanying drawings.